1) Field of the Invention
The present invention relates to an isolated type switching power supply apparatus whereby energy generated on the primary side is transferred to the secondary side in a non-contacted manner. The present invention can suitably be used in chargers used in instruments such as shavers, cellular phones, notebook-type personal computers, cordless electric equipment, etc.
Further, the present invention can preferably applied to power supplies for use in medical equipment where a leakage electric current into human bodies must be very small, or to power supplies where it is required to supply a stable power to a high voltage portion thereof, or to supply a power for opening and closing doors of, for instance, refrigerator or car, by combining the apparatus with a storage such as batteries or electric double layer capacitor.
2) Related Art Statement
FIG. 1 is a circuit diagram showing the construction of conventional switching power supply apparatuses. As shown in FIG. 1, an output of a DC power supply 1110 is switched by a switching element 1112 via a primary coil of a transformer 1111; then transferred to the secondary side of the transformer 1111 in accordance with an operation of the switching element 1112. The output of the secondary side of the transformer 1111 is rectified by a diode 1113 and smoothed by a capacitor 1114 to be outputted. The numerical reference 1101 represents a switching control circuit for the switching element 1112 and the numerical reference 1102 represents a driving circuit for the switching element 1112.
In this apparatus, a voltage detecting circuit 1103 is provided at the downstream of the capacitor 1114 to detect the output voltage on the secondary side of the transformer 1111; the detected voltage is sent to the primary side, on the basis of which the operation of the switching element 1112 is controlled. That is to say, when the output voltage is high the ON time of the switching element 1112 is controlled to be shorter, and when the output voltage is low it is controlled to be longer, so that the output voltage of the apparatus can be kept constant.
There are three conventional routes mentioned below as a means to transfer the voltage detected in the voltage detecting circuit 1103 to the primary side of the transformer 1111.
The first route is that so-called tertiary coil is provided in the transformer 1111 and the voltage appearing at the tertiary coil is assumed as the voltage of the secondary side of the transformer; then the switching element 1112 is controlled in accordance with the variation of the voltage at the tertiary coil. In FIG. 1, the voltage appearing at the tertiary coil 1111a is assumed as the voltage at the secondary side of the transformer 1111; an output of the tertiary coil 1111a is inputted to the switching control circuit 1101 after being rectified with the diode 1115 and being smoothed with the capacitor 1116. However, in such a construction, there is a problem that the voltage on the secondary side of the transformer 1111 is not correctly reflected at the tertiary coil 1111a. 
The second route is that: a PWM (Pulse Width Modulation) control circuit or a PFM (Pulse Frequency Modulation) control circuit is provided at the secondary side of the transformer and the output of the PWM or PFM control circuit is sent back to the primary side via another transformer to directly control the switching element. In FIG. 1, the output of the voltage detecting circuit 1103 is modulated by the PWM or PFM control circuit 1104; the pulse output of the circuit 1104 is sent to the driving circuit 1102 via a pulse transformer 1117, which is separately provided from the switching transformer 1111.
However, according to this construction, when the power supply 1110 is turn ON, the apparatus does not start up; therefore it is necessary to provide a pulse generating circuit 1106 on the primary side of the transformer 1111 for starting up the apparatus. Further, it is required to provide some circuit for re-starting the apparatus in a case where the switching operation is stopped due to the fact that an over current is generated on the load or the load is short-circuited.
The third route is that the output of the secondary side is transferred to the primary side via a photo-coupler. In FIG. 1, such an arrangement is shown that the output of the voltage detecting circuit 1103 is sent to the switching control circuit 1101 on the primary side via the photo-coupler 1105. However, according to this construction, it is sometimes difficult to conduct a correct controlling of the switching element because dirt adhering on the photo-coupler or the variation per hour of the photo-coupler per se causes the output of the photo-coupler 1105 to vary.